The present invention relates to a method for sealing and/or joining an end of a porous ceramic, preferably a ceramic filter element. More particularly, the present invention relates to a method for sealing and/or joining the end of a ceramic filter element by infiltrating metal into a portion of the filter element. The present invention also relates to filter elements produced by infiltrating metal into an end to seal the filter element, and to connecting devices for connecting such filter elements to a dissimilar material or together.
Filtration devices are used to separate two or more substances from each other on the basis of chemical or physical properties of the substances. Filtration devices can rely upon a chemical potential differential across a porous membrane for separating the substances. There has been significant interest in a range of filtration devices that can be categorized into groups including: (1) dense membranes in which materials diffuse through grain boundaries; (2) micropores (from about 3 to about 20 angstroms); (3) nano-filtration (from about 10 to about 80 angstroms); (4) ultra-filtration (from about 0.001 to about 0.2 micrometers); and micro-filtration (from about 0.2 to about 10.0 micrometers).
These filtration devices are particularly useful for separating substances from gas streams or liquids. A number of separation methods have been used in the various processing industries. The use of filtration devices having ceramic membranes is a relatively new area. The benefits of ceramic membranes generally include high-temperature capability, resistance to chemicals and good structural integrity which permits the separation membrane to be used under high pressures.
Ceramic membranes can be used for a wide variety of applications. For example, clarifying and sterilizing fruit juices and other liquids in the food and beverage industry; concentrating vaccines and enzymes or purifying amino acids and similar processes in the biotechnology industry; removing hydrogen from refinery streams and carbon dioxide and hydrogen sulfide from natural gas in the gas separation industry; separating oxygen from air; removing precipitated radionuclides and metal oxide and metal hydroxide particles from waste water; and purifying waters, acids, solvents and similar liquids in the electronic manufacturing industry.
Ceramic-based filter elements have been developed to take advantage of the properties of ceramic materials. For example, a filtration device having a ceramic filter element is disclosed in U.S. Pat. No. 4,069,157 by Hoover et al., which is incorporated herein by reference in its entirety. This patent discloses a filter element fabricated using a porous ceramic support, such as alumina (Al2O3) or cordierite (2MgOxc2x72Al2O3xc2x75SiO2), having a porosity of from about 30 percent to about 60 percent. A ceramic membrane layer is coated onto the interior channels of the porous ceramic support. The opening size in the membrane is controlled and can vary from about 0.002 micrometers up to about 1 micrometer.
Similar filtration devices utilizing ceramic filter elements are disclosed in the prior art. For example, see U.S. Pat. Nos. 4,894,160 and 4,971,696, both by Abe et al.; U.S. Pat. No. 4,983,423 by Goldsmith; or U.S. Pat. No. 4,981,590 by Van Tveen. Each of the foregoing patents is incorporated herein by reference in their entirety.
One of the problems associated with manufacturing filtration devices incorporating ceramic filter elements is the difficulties and limitations relating to sealing the end of the ceramic filter element. At least one end of the filter element must usually be sealed and prepared for installation into a filtration device by sealing the porous support and providing a surface that can easily be attached to the filtration device. Typically, the end of the filter must form a tight seal with a metal component, such as a stainless steel ring, to prevent the pressurized filtrant from bypassing the filter. Most filter elements have been sealed by internally sealing the porous ceramic with a ceramic slurry or cement and using organic materials, such as rubber or polymer xe2x80x9co-ringsxe2x80x9d to seal around the perimeter of the filter element. The o-rings are typically not capable of functioning in elevated temperatures and under corrosive conditions. Further, the o-rings will not form a tight seal around the ceramic if the perimeter of the ceramic filter element has defects or is xe2x80x9cout of round.xe2x80x9d This is a particularly acute problem when the filter is used to separate materials having very small diameters (e.g. ultrafiltration).
U.S. Pat. No. 5,203,488 by Wang et al. issued on Apr. 20, 1993. This patent is assigned to LANXIDE Technology Co. and is part of a series of patents assigned to LANXIDE Technology Co. that relate to ceramic-metal composites. Wang et al. disclose a method for joining two self-supporting bodies by a reactive infiltration process. It is disclosed that two materials can be bonded together utilizing the composite of the invention. For example, a powdered parent metal and a material which is to be reactively infiltrated can be placed between the two bodies. In an alternative embodiment, it is disclosed that an active brazing material can be placed between two composites formed according to the invention or may be placed between one body formed in accordance with the invention and a second body. It is disclosed that a foil, paste or powder which includes an active brazing alloy is placed between at least two self-supporting bodies made according to the first step of the invention.
The present invention is generally directed to a method for sealing and/or joining an end of a porous ceramic and preferably a ceramic-based filter element.
According to one aspect of the present invention, a method for sealing the end of a ceramic filter element is provided. The method can include the steps of providing a porous ceramic filter element having a first end and a second end and having filtering channels therethrough, contacting a portion of the first end of the filter element with a molten metal to infiltrate the metal into the first end of the filter element and cooling the infiltrated portion to form a filter element having a sealed end comprising a ceramic-metal composite portion.
According to certain embodiments of this aspect of the invention, the method can include the step of attaching a metal seal ring to the ceramic-metal composite portion. The step of attaching a metal seal ring to the ceramic-metal composite portion can include the step of brazing or welding a seal ring to the ceramic-metal composite portion. According to another embodiment, the ceramic filter element can include porous alumina having an open porosity of from about 30 volume percent to about 50 volume percent and the metal can include copper metal. The copper metal can also include an infiltration additive, such as oxygen. When oxygen is used as an infiltration additive, it is preferably added in an amount from about 1.5 weight percent to about 10 weight percent. The porous ceramic support can have an average pore size of from about 0.01 micrometers to about 2 millimeters, preferably from about 2 micrometers to about 15 micrometers, more preferably from about 6 micrometers to about 12 micrometers.
In another embodiment of this aspect of the invention, the method can further include the step of attaching a connecting means to the ceramic-metal composite portion. In one embodiment, such a connecting means permits sealable attachment of the porous ceramic element to a dissimilar material. In another embodiment, such a connecting means permits sealable attachment of one porous ceramic element to another porous ceramic element. In further embodiments, such a connecting means can include a swage fitting, a compression fitting, a weld, a braze, a metal bellows and/or threads which are formed on the ceramic metal composite portion.
According to another aspect of the present invention, a method for sealing the end of a ceramic-based filter element is provided. The method can include the steps of placing a metal in a refractory vessel, heating the metal to a temperature in excess of the melting temperature of the metal such that the metal is in the form of a molten pool having a top surface, contacting a cylindrical ceramic filter element having a first end and a second end and a plurality of channels therethrough with the top surface of the molten pool for a time sufficient to infiltrate a portion of the sintered ceramic filter element, removing the sintered ceramic filter element from contact with the molten pool, cooling the metal-infiltrated portion to form a ceramic filter element having a ceramic-metal composite portion and attaching a metal seal ring to the ceramic-metal composite portion.
According to certain embodiments of this aspect of the invention, the attaching step can include the step of brazing a seal ring to a ceramic-metal composite with a brazing alloy. The brazing alloy can include a copper-silver alloy. In one embodiment, the metal seal ring is a stainless steel ring. In other embodiments, the metal seal ring can be made from other metals, including but not limited to, Kovar(trademark) and carbon steel.
According to another aspect of the present invention, a method for sealing the end of a porous ceramic filter element is provided. This method can include the steps of providing a porous ceramic filter element having a first end and a second end and comprising a plurality of channels therethrough, providing a metal seal ring located around the perimeter of the first end of the filter element, contacting the first end of the filter element with a molten reactive braze metal to infiltrate a portion of the first end with the reactive braze metal and cooling the reactive braze metal to form a seal between the filter element and the seal ring. Preferably, the reactive braze metal is selected from the group comprising titanium, copper, nickel, silver and alloys thereof.
In yet another aspect of the present invention, a ceramic filter element for a filtration device is provided. The ceramic filter element includes a substantially cylindrical ceramic porous support having channels therethrough, a membrane layer coated on at least a portion of said channels in said porous ceramic support and a metal infiltrated into a portion of the cylindrical ceramic filter element to form a ceramic-metal composite therein.
In another embodiment of the present invention, the ceramic filter element has a tubular configuration. The tubular configuration is made up of a central channel surrounded by a porous sidewall. The substance to be filtered can be passed adjacent either the inner or outer wall of the tubular porous sidewall and the filtered material passes through the porous sidewall to the other side for collection.
According to certain embodiments of this aspect of the invention, the filter element includes alumina ceramic. The filter element can have an open porosity of from about 30 volume percent to about 50 volume percent. Further, the filter element can include a stainless steel ring attached to the ceramic-metal composite portion.
In other embodiments of this aspect of the invention, the filter element further includes a connecting means that is attached to the ceramic-metal composite portion. In further embodiments, such a connecting means can include a swage fitting, a compression fitting, a weld, a braze, a metal bellows and/or threads which are formed on the metal composite portion for screwing the ceramic filter element into a threaded member.